This invention relates to electrostatic copiers that produce an image by depositing a dry, powdered toner on paper. The invention relates specifically to a device for pressure fusing the toner image to the paper.
In most currently popular business office copying machines, the copying machine forms an optical image of the material to be copied which is projected onto the surface of a sensitized semiconductor, forming an electrostatic image. A dark color, electrostatically sensitive powdered material, called toner, is brought into contact with the sensitized semiconductor. Toner particles adhere to the areas on the semiconductor where an electrostatic charge is present. The toner particles, maintaining the pattern formed on the semiconductor, are then transferred to a sheet of paper. The toner particles are bonded to the paper, forming a permanent reproduction of the material being copied.
Three basic methods have been used to bond the toner to the paper: solvent fusing, thermal fusing, and pressure fusing. In addition, a combination of heat and pressure have been found to be effective also. Solvent fusing is least desirable because personnel can be exposed to harmful solvent vapors. Thermal fusing, or the combination of heat and pressure to fuse the toner, is the most commonly used technique. However, with any thermal based operation, power consumption and warm-up time are significant. Pressure-only fusing has not been used extensively because there have been no good pressure fixable toners that will operate with plain paper at pressures less than 200-300 pounds per linear inch (pli). At these pressures, the size, weight and cost of the pressure applying elements become significant factors in a copying machine.
In most prior art electrostatic copiers that use pressure fusing devices to fix or fuse the toner image to paper, the paper with an unfused image of loosely adhering toner particles is passed between two parallel rolls that are pressed together. Generally, the rolls are supported at their ends on bearings and usually the rolls are of equal diameter. Most often, only one of the rolls is driven, the second roll being an idler roll. Accordingly, the surface velocity of the rolls is the same. In order to achieve essentially uniform fusing pressures along their entire length, the rolls are of relatively large diameter so that their deflection can be low.